Note that this use of { embedded: 'always' } is unrelated to
the { embedded: 'always' } that is defined as an option on DS.attr as part of
defining a model while working with the ActiveModelSerializer. Nevertheless,
using { embedded: 'always' } as an option to DS.attr is not a valid way to setup
embedded records.

The attrs option for a resource { embedded: 'always' } is shorthand for:

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{
serialize: 'records',
deserialize: 'records'
}

Configuring Attrs

A resource's attrs option may be set to use ids, records or false for the
serialize and deserialize settings.

The attrs property can be set on the ApplicationSerializer or a per-type
serializer.

In the case where embedded JSON is expected while extracting a payload (reading)
the setting is deserialize: 'records', there is no need to use ids when
extracting as that is the default behavior without this mixin if you are using
the vanilla EmbeddedRecordsMixin. Likewise, to embed JSON in the payload while
serializing serialize: 'records' is the setting to use. There is an option of
not embedding JSON in the serialized payload by using serialize: 'ids'. If you
do not want the relationship sent at all, you can use serialize: false.

EmbeddedRecordsMixin defaults

If you do not overwrite attrs for a specific relationship, the EmbeddedRecordsMixin
will behave in the following way:

Model Relationships

Embedded records must have a model defined to be extracted and serialized. Note that
when defining any relationships on your model such as belongsTo and hasMany, you
should not both specify async: true and also indicate through the serializer's
attrs attribute that the related model should be embedded for deserialization.
If a model is declared embedded for deserialization (embedded: 'always' or deserialize: 'records'),
then do not use async: true.

To successfully extract and serialize embedded records the model relationships
must be setup correcty. See the
defining relationships
section of the Defining Models guide page.

Records without an id property are not considered embedded records, model
instances must have an id property to be used with Ember Data.

Example JSON payloads, Models and Serializers

When customizing a serializer it is important to grok what the customizations
are. Please read the docs for the methods this mixin provides, in case you need
to modify it to fit your specific needs.